1. Field of the Invention
This invention relates to a method of and an apparatus for obtaining a fluorescence image on the basis of fluorescence emitted from a specimen such as an organic body upon excitation by exciting light.
2. Description of the Related Art
There has been studied a fluorescence diagnosis system in which exciting light is projected onto an organic body or the like, and information useful for diagnosis is obtained from the intensity or spectral band intensity of fluorescence emitted from the organic body upon excitation by the exciting light. Such a fluorescence diagnosis system includes, for instance, those in which autofluorescence emitted from an organic tissue upon excitation by the exciting light is detected, and those in which fluorescence emitted from an organic tissue which has absorbed a drug for fluorescence diagnosis is detected. Such a fluorescence diagnosis system is generally incorporated in an instrument such as an endoscope, a colposcope, or an operative microscope which is inserted into a body cavity and is used for analyzing conditions of the organic tissue.
At the beginning, an attempt to make a diagnosis on the basis of the intensity of fluorescence emitted from an organic tissue upon excitation by exciting light was made. However, as the relative position between the tissue and the part of the system from which the exciting light is projected onto the tissue such as the distance, angle and the like therebetween changes, the intensity of the exciting light projected onto the organic tissue changes, which results in change in the intensity of the fluorescence emitted from the tissue. Accordingly, the same site of the organic body can emit fluorescence of different intensities depending upon the position of the exciting light projecting part of the fluorescence diagnosis system, which shows that a sufficient diagnostic performance cannot be obtained from the intensity of the fluorescence by itself. Therefore, recently there have been made various attempts to recognize change in the tissue condition on the basis of the fact that the profile of spectral band intensities of the fluorescence changes with change in the tissue condition. For example, a diseased tissue greatly differs from a normal tissue in the ratio of intensity of a green region wavelength component of the fluorescence to intensity of a red region wavelength component of the same. There has been proposed in Japanese Unexamined Patent Publication No. 6(1994)-54792 a fluorescence diagnosis system in which the ratio of intensity of a green region wavelength component to intensity of a red region wavelength component of autofluorescence emitted from an organic tissue to be diagnosed is compared with that of an organic tissue which has been determined to be normal by a different system and whether the organic tissue to be diagnosed is diseased or normal is determined on the basis of the comparison.
Further, we have proposed in Japanese Unexamined Patent Publication No. 10(1998)-225436 a fluorescence diagnosis system in which the intensity in a green wavelength range of fluorescence emitted from an organic tissue is normalized with the intensity of the overall fluorescence (substantially over the entire wavelength range of the spectral bands of the fluorescence) and is compared with that of an organic tissue which has been determined to be normal by a different system, and whether the organic tissue is diseased or normal is determined on the basis of comparison.
However, the intensity of fluorescence of each pixel of an image obtained by taking weak fluorescence emitted from the organic tissue includes noise inherent to the image taking device (e.g., fixed pattern noise, photon shot noise, dark shot noise and read-out noise), noise inherent to the electric processing circuit, noise inherent to the signal transfer system, noise inherent to the optical system (noise due to stray light generated, for instance, by light scattered by dirt or the like adhering to the optical components) and the like, and the intensity of the noise is too large as compared with the intensity of the fluorescence especially for pixels in a region where the intensity of fluorescence is very weak. Accordingly, when the intensity in a green wavelength range of fluorescence emitted from an organic tissue for pixels in a region where the intensity of fluorescence is very weak is normalized by the intensity in a green wavelength range by the intensity of the overall fluorescence, the information obtained by the normalization cannot accurately reflect the condition of the organic tissue since the former intensity and the latter intensity are both very weak and the intensity of the noise components included in the former and latter intensities is too large as compared with the former and latter intensities. When information on such pixels are used as it is in reproduction of an image, the pixels can visually adversely affect observation of the reproduced image.
These problems are common to the fluorescence diagnosis systems in which autofluorescence emitted from an organic tissue upon excitation by the exciting light is detected, and those in which fluorescence emitted from an organic tissue which has absorbed a photosensitive substance for fluorescence diagnosis such as ATX-S10, 5-ALA, Npe6, HAT-D01 or Photofrin-2 (will be referred to as xe2x80x9cfluorescence produced by a photosensitive substancexe2x80x9d hereinbelow) is detected.
In view of the foregoing observations and description, the primary object of the present invention is to provide a method of and an apparatus for obtaining a fluorescence image on the basis of fluorescence emitted from a specimen such as an organic body upon excitation by exciting light in which even if adverse pixels, whose intensities include noise in a large proportion, are included in pixels obtained by taking fluorescence emitted from the specimen upon excitation by the exciting light, a fluorescence image which cannot be visually adversely affected by existence of the adverse pixels can be obtained.
In accordance with one aspect of the present invention, there is provided a method of obtaining a fluorescence image comprising the steps of projecting exciting light onto a specimen such as an organic body, detecting an intensity in at least one wavelength range of fluorescence emitted from the specimen upon excitation by the exciting light, and obtaining image data representing a fluorescence image of the specimen through an operation based on the intensity of the fluorescence, wherein the improvement comprises the steps of
determining whether each of pixels forming the fluorescence image is adequate for said operation on the basis of the intensity of the fluorescence of each pixel, and carrying out said operation on adequate pixels, which have been determined to be adequate for said operation, to obtain values for the adequate pixels while allotting to inadequate pixels, which have been determined not to be adequate for said operation, values which will not visually adversely affect the part of the fluorescence image corresponding to the adequate pixels.
For example, whether each of pixels is adequate for said operation may be determined by comparing the intensity of fluorescence for the pixel with the value of noise for the pixel which is generated by means for measuring the intensity of fluorescence and has been measured and stored in advance.
When carrying out image processing on the image data, the image processing may be carried out only on the adequate pixels.
The specimen may be an organic body and the fluorescence may be autofluorescence.
The exciting light may be emitted from a GaN semiconductor laser.
In accordance with another aspect of the present invention, there is provided an apparatus for obtaining a fluorescence image comprising an exciting light projecting means for projecting exciting light onto a specimen such as an organic body, a fluorescence intensity measuring means for measuring an intensity in at least one wavelength range of fluorescence emitted from the specimen upon excitation by the exciting light, and an operational processing means for obtaining image data representing a fluorescence image of the specimen through an operation based on the intensity of the fluorescence, wherein the improvement comprises that
a determining means determines whether each of pixels forming the fluorescence image is adequate for said operation on the basis of the intensity of the fluorescence of each pixel, and said operational processing means carries out said operation on adequate pixels, which have been determined to be adequate for said operation, to obtain values for the adequate pixels and allots to inadequate pixels, which have been determined not to be adequate for said operation, values which will not visually adversely affect the part of the fluorescence image corresponding to the adequate pixels.
For example, the determining means may determine whether each of pixels is adequate for said operation by comparing the intensity of fluorescence for the pixel with the value of noise for the pixel which is generated by the fluorescence intensity measuring means itself and has been measured and stored in advance.
The apparatus may further comprise an image processing means and the image processing means may carry out the image processing only on the adequate pixels.
The specimen may be an organic body and the fluorescence may be autofluorescence.
The exciting light projecting means may comprise a GaN semiconductor laser as an exciting light source.
The apparatus may be, for instance, an endoscope.
The xe2x80x9cnoise for the pixel which is generated by the fluorescence intensity measuring means itselfxe2x80x9d means a part or the whole of noise generated by the fluorescence intensity measuring means between the time the fluorescence intensity measuring means starts to receive the fluorescence and the time it finishes obtaining intensities of the fluorescence in respective wavelength ranges, or noise containing therein a part or the whole of such noise. Typical components of the noise generated by the fluorescence intensity measuring means include noises generated in the circuit of the image taking device, the circuit handling electric signals obtained by the image taking device, the signal transfer circuit for transferring the signals, and the optical system for taking the fluorescence.
When the fluorescence intensity measuring measures an intensity of the fluorescence in only one wavelength range, and the operational processing means obtains image data representing a fluorescence image of the specimen through an operation based on the intensity of the fluorescence in the wavelength range, the operation includes, for instance, an operation for allotting to pixels values reflecting the intensity of the fluorescence and an operation carried out between the intensity of the fluorescence in the wavelength range and a reference intensity obtained from light reflected by the specimen when a near infrared ray is projected onto the specimen. When the fluorescence intensity measuring measures intensities of the fluorescence in two or more wavelength ranges, and the operational processing means obtains image data representing a fluorescence image of the specimen through an operation based on the intensities of the fluorescence in the wavelength ranges, the operation includes, for instance, an operation carried out between the intensities of the fluorescence and an operation carried out among the intensities of the fluorescence in the wavelength ranges and a reference intensity obtained from light reflected by the specimen when a near infrared ray is projected onto the specimen.
When the amount of fluorescence received by a pixel is very small and accordingly the amount of noise included in the value for the pixel is too large relative to the amount of the fluorescence for the operation to provide a value which correctly reflects the condition of the specimen, the pixel is determined to be an inadequate pixel.
On the other hand, when the amount of fluorescence received by a pixel is large and accordingly the amount of noise included in the value for the pixel is relatively small so that the operation can provide a value which correctly reflects the condition of the specimen, the pixel is determined to be an adequate pixel.
The adequate and inadequate pixels do not exist from the beginning but are set on the basis of the amount of noise generated by the fluorescence intensity measuring means by itself and the measured amount of fluorescence.
Auto fluorescence emitted from the inside of an organic body is sometimes called xe2x80x9cin vivo autofluorescencexe2x80x9d.
In accordance with the present invention, since values which will not visually adversely affect the part of the fluorescence image corresponding to the adequate pixels are allotted to inadequate pixels, a fluorescence image which is free from adverse influence of inadequate pixels and is excellent in quality can be obtained.
Further, since the operation is carried out only on adequate pixels, the operation time can be shortened.
When whether each of pixels is adequate for said operation may be determined by comparing the intensity of fluorescence for the pixel with the value of noise for the pixel which is generated by means for measuring the intensity of fluorescence and has been measured and stored in advance, adequate pixels can be more rationally distinguished from inadequate pixels.
When image processing is carried out only on the adequate pixels, a fluorescence image which is free from adverse influence of inadequate pixels can be reproduced and the processing time can be shortened.
Use of a GaN semiconductor laser as the exciting light source contributes to miniaturization of the apparatus and reduction of the cost.
The method and apparatus of the present invention can be used to observe the condition of the inside of an organic body through autofluorescence emitted from the organic body upon excitation by the exciting light. That is, the present invention can be applied to an endoscope.